In the science of microbiology, which by its very definition is that branch of knowledge which concerns living micro-organisms, there is a never-ending need to observe, study, catalog and learn about bacteria, how they multiply, the size and operation of their colonies, their incubation periods, etc. There frequently arises, either in scientific microbiologic inquiries or in the field of medicine (e.g., for antibiotic sensitivity assay), the need to determine the total number of bacteria present in a sample, or the number of bacterial colonies which have incubated.
The normal procedure for determining the quantity of bacteria in an unknown solution, known as the pour plate method, is complex, involving making serial dilutions with multiple pipetting, and mixing each dilution with sterile liquid agar. A known aliquot of each dilution is mixed with an agar nutritive media in a Petri dish. This must all be done under sterile conditions to avoid the introduction of outside bacteria. The agar makes a gel and holds the bacteria in place. Upon incubation the bacteria reproduce sufficiently to form visible colonies which can be counted from one of the dilutions. The plates which contain too many or too few bacterial colonies are discarded and the one containing a countable number is visually counted. Knowing the number counted and the dilution from which the plate was made, the bacterial concentration in the original material may be calculated.